Praise be to Allah.
The father is obliged to get his son married if the father is rich and the son is poor and cannot afford to get married, because marriage is part of the obligatory spending. Allaah says (interpretation of the meaning):
“but the father of the child shall bear the cost of the mother’s food and clothing on a reasonable basis”
So the father has to get his sons married if he is able to and they cannot afford it. He has to spend on them with regard to food, drink, education etc, because spending is obligatory upon the father.
The son does not have to spend on his father unless two conditions are met:
1 – The son is rich (i.e., he has what he needs and more)
2 – The father is poor.
If these two conditions are met, then the son is obliged to spend on his father. Here the questioner mentions that her father-in-law is a doctor and that his income is good. If that is the case then the son is not obliged to spend on him. But if the father asks his son for something, even if he is rich, and the son does not need that wealth and will not be affected by giving it to his father, then he should give it to his father lest he be regarded as being disobedient towards his father. But if the son needs that money, he does not have to give it to his father; he should explain to his father the financial difficulty that he is facing [?} and that after he had paid off his debts and has become financially stable he will send money to his father as much as he can.
“Allaah burdens not a person beyond his scope”
[al-Baqarah 2:286 – interpretation of the meaning]
And Allaah knows best.